This application claims the priority of German Patent Application No. 197 54 012.0, filed Dec. 5, 1997, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to a system for the water vapor reforming of a hydrocarbon. Systems of this type are used, for example, in mobile applications such as in fuel-cell operated motor vehicles for the water vapor reforming of methanol carried along in liquid form in order to provide the hydrogen needed for the fuel cells without requiring a large hydrogen storage device. For this use, a compact construction of the system is required that can be implemented with a relatively low weight and expenditures that are as low as possible. Furthermore, fast reacting of the system to load changes is desirable for motor vehicle applications, for which a compact construction is also favorable. Other desirable characteristics of mobile systems are a high efficiency and low control and automatic control expenditures, whereby a high system reliability can be achieved.
Systems for the water vapor reformation of hydrocarbon are known in which certain system components are integrated into a respective joint constructional part for achieving a compact construction. Thus, Japanese Laid-Open Patent Applications JP 62138306 A, JP 63021203 A and JP 63040701 A describe reforming systems in which the reformer carrying out the reforming reaction and an evaporator connected in front of it are integrated into a common reactor component. A burner is also assigned to the common reactor component in which fuel is burned while igniting to directly heat the evaporator. In addition, the burner may heat the reformer by means of its hot combustion exhaust gases.
In the reforming system disclosed in U.S. Pat. No. 5,516,344, the reformer, together with a CO shift converter connected behind it, are integrated in a common component to which a burner is assigned that burns a fed combustible mixture while igniting. The hot combustion exhaust gases will then, among others, heat the reformer and the CO shift converter.
In Japanese Laid-Open Patent Application JP 07126001 A, a system is described that contains a modular reactor unit of the plate stack type. In an integrated manner, this modular reactor unit contains an evaporator, a reformer and a CO-oxidant. These three system components are serially arranged in a transverse stack direction behind one another in the form of a first group of alternating, plate layers. A burner is connected in front of the evaporator, in which burner a fed mixture is burned while igniting. The hot combustion exhaust gases are guided in parallel to the reforming gas flow through a second group of alternating plate layers forming a heat exchanger structure that alternate with the plate layers of the first group and in the process heat the evaporator, the reformer and the CO-oxidant.
U.S. Pat. No. 4,746,329 discloses a methanol reforming reactor having a cylindrical construction and consisting of several radially successive annuli. On the bottom side of the reactor cylinder, a burner unit is situated that may be formed by a catalytic burner. The hot burner exhaust gases are guided through the radially most exterior annulus upwards and are then deflected into the radially interior adjacent annulus, where they are in a thermal contact with a reforming annulus adjoining radially on the inside. In this case, an upper part of the reforming space extends beyond the exterior annuli carrying the combustion gas so that a lower operating temperature exists in this area. This cooler upper reforming space area is used as a CO shift unit. On the inside, the reforming space is adjoined by an evaporator annulus which, radially toward the inside by way of a cylindrical wick, adjoins an interior tempering space, into which the combustion gases are deflected after a downward flow through the second exterior annulus in a lower cylinder area. A hydrogen containing anode gas of a fuel cell system is used as the fuel for the burner unit. The combustion exhaust gases therefore contain water vapor, of which, after the exiting of the combustion exhaust gases on the upper cylinder face, at least a portion is fed to the evaporator.
German Published Patent Application DE 38 03 080 Al discloses a reforming system for generating synthesis gases containing hydrogen, carbon monoxide and carbon dioxide from hydrocarbon-containing charged substances as well as an operating process therefor. The charged substances are first subjected to an at least one-stage primary reforming; then to a partial oxidation; and subsequently to another secondary reforming; and finally to a carbon monoxide conversion. In this case, the waste heat of the exothermal carbon monoxide conversion is used for the primary vapor reforming, for the purpose of which the corresponding primary reforming stage and the CO conversion stage are in a thermal contact by way of a heat-conducting separating wall.
The present invention is based on the technical problem of providing a system that has a comparatively high efficiency and comparatively high dynamics that is sufficient for use in fuel-cell-operated motor vehicles at low control and automatic control expenditures, and which has a very compact construction and can be built at relatively low expenditures.
The present invention solves this problem by providing a system that includes a modular reactor unit of the plate stack type, of the tube bundle type or of a combination of these two types. The modular reactor unit contains at least one evaporator, a prereforming unit, a main reformer, a CO removal unit and a catalytic burner unit in an integrated form. The presence of these components results in a high-efficiency reforming conversion, in which the concentration of the carbon monoxide contained in the reformate gas can be limited to a desired value by means of the CO removal unit. The integrated formation of these system components in the common modular reactor unit provide the prerequisite for high dynamics of the system so that it can satisfactorily react to load changes, for example, typical load changes for. motor vehicle operation. The operating performance of the system may be further advantageously influenced in that (1) the evaporator and the main reformer are in a thermal contact with the catalytic burner unit and, (2) the prereforming unit is in a thermal contact with the CO removal unit, in each case by way of a heat-conducting separating medium.
In another embodiment of the present invention, the burner unit contains at least two catalytic burners. One catalytic burner is in thermal contact with the evaporator and the other catalytic burner is in thermal contact with the main reformer, each forming one module respectively having a heat exchanger structure. The module construction for the evaporator/burner component and the main reformer/burner component promotes a flexible modular construction of the reactor unit.
In another embodiment of the present invention, the prereforming unit, like the CO removal unit, also comprises two stages. The CO removal unit comprises a CO shift stage and a CO oxidation stage that is connected behind it. The CO oxidation stage together with the first prereforming stage and the CO shift stage together with the second prereforming stage each form one module respectively having a heat exchanger structure. This modular construction again promotes a compact modular construction of the modular reactor unit and thus of the system as a whole. In a further development of this modular construction, the corresponding modules (i.e., evaporator/burner; main reformer/burner; CO oxidation stage/first prereforming stage; and CO shift stage/second prereforming stage) are arranged side-by-side. Thermally insulating separating elements may be provided between respective successive modules.
In another embodiment of the present invention, heating ducts are provided in the oxidation stage/prereforming module and/or in the shift stage/prereforming module through which the hot combustion exhaust gas of the catalytic burner unit can be guided in order to actively heat these modules.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.